1. You need to turn off the cold water supply to the tank in the loft. If this doesn't have an isolation valve, you'll probably have to shut your main incoming stop cock. 2. Fully open the hot tap with the problem and wait until all water stops coming out of it. You can then change the tap washer.
Note: If there is a valve in the pipe between the bottom of the cold water tank in the loft and the hot water copper cylinder, simply close this instead of step 1 above. Then do step 2.
It may be worth getting hold of some isolation valves before you start the job so that you can fit them to various hot taps at the same time as fitting the washer. If you have to go down the route of turning off the main stop cock, you can fit them to cold taps as well.
For information, the hot water cylinder itself will remain almost full because the feed to the hot taps is taken from the top of it. Once the cold water tank that feeds it empties, or if the supply from the cold water tank to the cylinder is shut off, no more water can be pushed out of the cylinder to the hot taps.
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